An electrolytic capacitor has a small size and a large capacitance, can be produced at a low cost and exhibits excellent properties in smoothening of rectified output. It is one of important elements constituting various electric and electronic devices.
In general, an electrolytic capacitor comprises as an anode a so-called valve metal such as aluminum and tantalum having an oxide layer formed thereon as a dielectric layer. A cathode is disposed opposed to the anode with a separator provided interposed therebetween. An electrolytic solution is held in the separator.
The formed foil which acts as anode is prepared by etching a foil made of a high purity valve metal to increase the surface area thereof, and then applying a voltage to the foil in an electrolytic solution to form an oxide layer thereon. The withstand voltage of the oxide layer as dielectric material is determined by the voltage applied during the formation. The cathode foil as cathode is made of an etched high purity foil.
The separator prevents the anode and the cathode from being short-circuited to each other and holds an electrolytic solution. As the separator there is used a thin low density paper such as kraft paper and manila paper.
The anode foil having a tab connected thereto and the cathode foil having a tab connected thereto are then laminated with the separator provided interposed therebetween. The laminate is then wound to prepare a capacitor element. The capacitor element is then impregnated with an electrolytic solution. The capacitor element is inserted into a case which is then hermetically sealed. The anode foil is then subjected to reformation to prepare an electrolytic capacitor.
As previously mentioned, the electrolytic solution for electrolytic capacitor is brought into direct contact with the dielectric layer to act as a true cathode. In other words, the electrolytic solution is provided interposed between the dielectric material and the collector cathode in the electrolytic capacitor. This means that the resistivity of the electrolytic solution is inserted in series with the electrolytic capacitor. Accordingly, the electrical conductivity of the electrolytic solution has an effect on the magnitude of the dielectric loss of the capacitor. The voltage at which the aluminum foil undergoes shortcircuiting in the electrolytic solution is called sparking voltage of electrolytic solution. It indicates the oxide film-forming properties of electrolytic solution.
Boric acid and organic dicarboxylic acids such as sebacic acid and azelaic acid have heretofore been used as electrolytic solutions for middle and high voltage because they can provide a relatively high sparking voltage. In some cases, butyloctanedioic acid, which provides a high sparking voltage and exhibits a high electrical conductivity, is used as a solute (JP-B-60-13296 (The term "JP-B" as used herein means an "examined Japanese patent publication")). In some recent cases, 1,7-octanedicarboxylic acid is used (JP-A-2-224217 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")).
However, in recent years, electronic apparatus comprising a switching power supply have been widely used in the home. Thus, there is a growing demand for the safety of electrolytic capacitors. In other words, the switching power supply incorporated in these electronic apparatus comprises an electrolytic capacitor. In a working atmosphere where the electric power supplied is unstable, an excess voltage may be applied to the electrolytic capacitor. Thus, there is a growing demand for a high safety electrolytic capacitor which can withstand the excess voltage. However, electrolytic capacitors comprising an electrolytic solution as previously mentioned cannot meet this demand. It has thus been desired to provide electrolytic capacitors having a higher withstand voltage and overvoltage resistance.
Further, electronic apparatus such as invertor having an improved efficiency have been demanded. To this end, these apparatus have been able to operate at a higher frequency. In order to cope with this tendency, the electrolytic capacitors used must accordingly operate at a higher frequency. The electrolytic capacitors comprising an electrolytic solution as previously mentioned cannot meet this demand, too. It has thus been desired to provide electrolytic capacitors having a lower dielectric loss.